Image forming apparatus

ABSTRACT

An image forming apparatus that forms an image according to light emitted from a light source is provided. The image forming apparatus includes: a first image processor that performs image processing on image data having a first resolution and outputs the resulting image data; a resolution converter that acquires the image data having the first resolution output from the first image processor and converts the image data to image data having a second resolution that is higher than the first resolution; a modulation signal generator that modulates the image data having the second resolution according to a clock signal to thereby generate a modulation signal; a light source driver that drives the light source according to the modulation signal; and a second image processor that performs image processing on the image data having the second resolution to be modulated to the modulation signal.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority to and incorporates by referencethe entire contents of Japanese Patent Application No. 2013-054368 filedin Japan on Mar. 15, 2013.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an image forming apparatus.

2. Description of the Related Art

Digital printers employing an electrophotographic process have latelybecome widely used in the production printing field. The digitalprinters employing the electrophotographic process are thus required tooffer higher image quality and greater reliability. The digital printersemploying the electrophotographic process are particularly required tooffer, for example, improved fine line reproducibility, improvedcharacter reproducibility (e.g., improved reproducibility of charactersof minute sizes corresponding to 2 to 3 points), inhibition ofcharacters from becoming broader due to the electrophotographic process,and improved color shift correction accuracy.

In order to achieve the higher image quality, the digital printeremploying the electrophotographic process includes an image processorthat corrects image data through image processing. The image processorperforms image processing for, for example, multi-bit data having a highresolution of 1200 dots per inch (dpi) or 2400 dpi.

The digital printer employing the electrophotographic process furtherincludes, for example, a photosensitive drum, a light source, a polygonmirror, and a scanning optical system. Specifically, the photosensitivedrum has a surface that functions as a scanned surface havingphotosensitivity. The light source emits a laser beam. The polygonmirror deflects the laser beam from the light source. The scanningoptical system guides the laser beam deflected by the polygon mirroronto the surface (scanned surface) of the photosensitive drum. Thedigital printer employing the electrophotographic process modulates thelight beam emitted from the light source according to the image data tothereby irradiate the scanned surface with the light beam from the lightsource. And by scanning the scanned surface with the light beam, thedigital printer employing the electrophotographic process forms anelectrostatic latent image on the photosensitive drum according to theimage data.

The digital printer employing the electrophotographic process having theconfiguration as described above includes as the light source a laserdiode array (LDA), a vertical-cavity surface-emitting laser (VCSEL), orother element having a plurality of light emitting points. This enablesthe digital printer employing the electrophotographic process to form anelectrostatic latent image having a resolution higher than image data of1200 dpi, specifically, a 2400-dpi or 4800-dpi electrostatic latentimage.

Japanese Patent Nos. 4968902 and 4640257 each disclose a technique inwhich, through processing performed by an image processor, outlinedportions in the image are detected and outlines are extended or pixelsaround white-on-black inverted characters are corrected. Thereby,inverted characters are prevented from being collapsed and improvedcharacter reproducibility is achieved. Japanese Patent No. 4912071discloses an arrangement in which a light source drive circuit includesa light source modulation signal generating circuit that corrects bendand skew in a scanning line (a locus of a light beam deflected by apolygon mirror).

Processing of a high-density image involves a problem in data transferfrom the image processor to the light source drive circuit downstreamthereof. If the image processor processes multi-bit data images with aresolution, for example, of 2400 dpi or 4800 dpi, the degree of freedomin image processing is enhanced and reproducibility of 1200-dpicharacters and lines of minute sizes can be improved. In high-densityimage processing, however, an enormous amount of data needs to betransferred from the image processor to the downstream light sourcedrive circuit, which is a bottleneck in productivity.

If the correction is made with the light source modulation signalgenerating circuit of the downstream light source drive circuit as inJapanese Patent No. 4912071, the amount of data transferred from theimage processor to the light source drive circuit does not increase. Thedata transferred to the light source drive circuit is, however,converted to light source ON/OFF information, which makes it difficultto perform complicated corrections.

In view of the foregoing situation, there is a need to provide an imageforming apparatus capable of performing image processing at highresolutions without increasing an image data transfer amount.

SUMMARY OF THE INVENTION

It is an object of the present invention to at least partially solve theproblems in the conventional technology.

According to the present invention, there is provided an image formingapparatus that forms an image according to light emitted from a lightsource, the image forming apparatus comprising: a first image processorthat performs image processing on image data having a first resolutionand outputs the resulting image data; a resolution converter thatacquires the image data having the first resolution output from thefirst image processor and converts the image data to image data having asecond resolution that is higher than the first resolution; a modulationsignal generator that modulates the image data having the secondresolution according to a clock signal to thereby generate a modulationsignal; a light source driver that drives the light source according tothe modulation signal; and a second image processor that performs imageprocessing on the image data having the second resolution to bemodulated to the modulation signal.

The above and other objects, features, advantages and technical andindustrial significance of this invention will be better understood byreading the following detailed description of presently preferredembodiments of the invention, when considered in connection with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram illustrating the configuration of a colorprinter 2000 according to an embodiment of the present invention;

FIG. 2 is a diagram illustrating an exemplary arrangement of opticalsensors 2245 a, 2245 b, 2245 c;

FIG. 3 is a diagram illustrating the configuration of the opticalsensors 2245 a, 2245 b, 2245 c;

FIG. 4 is a diagram illustrating the configuration of an optical systemof an optical scanning device 2010;

FIG. 5 is a diagram illustrating an exemplary optical path from a lightsource 2200 a to a polygon mirror 2104 and an exemplary optical pathfrom a light source 2200 b to the polygon mirror 2104;

FIG. 6 is a diagram illustrating an exemplary optical path from a lightsource 2200 c to the polygon mirror 2104 and an exemplary optical pathfrom a light source 2200 d to the polygon mirror 2104;

FIG. 7 is a diagram illustrating an exemplary optical path from thepolygon mirror 2104 to photosensitive drums 2030;

FIG. 8 is a diagram illustrating the configuration of an electricalsystem of the optical scanning device 2010;

FIG. 9 is a diagram illustrating the configuration of an interface unit3101;

FIG. 10 is a diagram illustrating the configuration of an imageprocessing unit 3102;

FIG. 11 is a diagram illustrating the configuration of a drive controlunit 3103;

FIG. 12A is a diagram illustrating an exemplary 5-point white-on-blackinverted character and exemplary enlarging steps in units of 1200 dpi;

FIG. 12B is a diagram illustrating an exemplary 3-point white-on-blackinverted character and exemplary enlarging steps in units of 4800 dpi;

FIG. 13A is a diagram illustrating exemplary thinning steps in units of1200 dpi;

FIG. 13B is a diagram illustrating exemplary thinning steps in units of4800 dpi;

FIG. 14 is a diagram illustrating exemplary smoothing steps; and

FIG. 15 is a diagram illustrating a modification of the drive controlunit 3103 of the optical scanning device 2010.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A color printer 2000 as an exemplary image forming apparatus accordingto an embodiment of the present invention will be described in detailwith reference to the accompanying drawings. FIG. 1 is a schematicdiagram illustrating the configuration of the color printer 2000according to the embodiment.

The color printer 2000 is a tandem type multi-color printer that forms afull-color image by superimposing four colors (black, cyan, magenta, andyellow), one on top of another.

The color printer 2000 includes an optical scanning device 2010, fourphotosensitive drums 2030 a, 2030 b, 2030 c, 2030 d (to be genericallyreferred to as a photosensitive drum 2030), four cleaning units 2031 a,2031 b, 2031 c, 2031 d (to be generically referred to as a cleaning unit2031), and four charging devices 2032 a, 2032 b, 2032 c, 2032 d (to begenerically referred to as a charging device 2032). The color printer2000 further includes four developing rollers 2033 a, 2033 b, 2033 c,2033 d (to be generically referred to as a developing roller 2033) andfour toner cartridges 2034 a, 2034 b, 2034 c, 2034 d (to be genericallyreferred to as a toner cartridge 2034). The color printer 2000 stillfurther includes a transfer belt 2040, a transfer roller 2042, fixingrollers 2050, a feed roller 2054, a pair of registration rollers 2056,discharging rollers 2058, a paper feed tray 2060, a discharge tray 2070,a communication control device 2080, a density detector 2245, four homeposition sensors 2246 a, 2246 b, 2246 c, 2246 d (to be collectivelyreferred to as a home position sensor 2246), and a printer controldevice 2090.

The communication control device 2080 controls bi-directionalcommunications with a host device (e.g., a computer) via, for example, anetwork.

The printer control device 2090 generally controls different elements ofthe color printer 2000. The printer control device 2090 includes, forexample, a central processing unit (CPU), a ROM that stores therein acomputer program described in codes to be executed by the CPU andvarious types of data used for executing the program, a RAM that servesas a working memory, and an AD converter circuit that converts analogdata to the corresponding digital data. The printer control device 2090,while controlling each of the different elements according to a requestfrom the host device, transmits image data from the host device to theoptical scanning device 2010.

The photosensitive drum 2030 a, the charging device 2032 a, thedeveloping roller 2033 a, the toner cartridge 2034 a, and the cleaningunit 2031 a are used as one unit. These elements constitute an imageforming station to form a black image (may be referred to as a Kstation).

The photosensitive drum 2030 b, the charging device 2032 b, thedeveloping roller 2033 b, the toner cartridge 2034 b, and the cleaningunit 2031 b are used as one unit. These elements constitute an imageforming station to form a cyan image (may be referred to as a Cstation).

The photosensitive drum 2030 c, the charging device 2032 c, thedeveloping roller 2033 c, the toner cartridge 2034 c, and the cleaningunit 2031 c are used as one unit. These elements constitute an imageforming station to form a magenta image (may be referred to as an Mstation).

The photosensitive drum 2030 d, the charging device 2032 d, thedeveloping roller 2033 d, the toner cartridge 2034 d, and the cleaningunit 2031 d are used as one unit. These elements constitute an imageforming station to form a yellow image (may be referred to as a Ystation).

The photosensitive drum 2030 has a photosensitive layer formed on itssurface. Specifically, the surface of the photosensitive drum 2030assumes a scanned surface. The photosensitive drums 2030 a, 2030 b, 2030c, 2030 d each have a rotational axis extending in parallel with eachother and each rotate, for example, in an identical direction (e.g., inthe direction indicated by the arrowed line in plane in FIG. 1).

The following description is based on a viewer's perspective in which,in an XYZ three-dimensional Cartesian coordinate system, a directionextending in parallel with the central axis of the photosensitive drum2030 is the Y-axis direction and a direction in which the photosensitivedrums 2030 are arrayed is the X-axis direction.

The charging device 2032 uniformly charges the surface of thephotosensitive drum 2030. The optical scanning device 2010 irradiatesthe charged surface of the photosensitive drum 2030 with a light beammodulated for each color based on the image data (black image data, cyanimage data, magenta image data, yellow image data). As a result, on thesurface of the photosensitive drum 2030, an electric charge is erasedonly on portions irradiated with the light and a latent imagecorresponding to the image data is formed on the surface of thephotosensitive drum 2030. The latent image thus formed moves toward thedeveloping roller 2033 as the photosensitive drum 2030 rotates. Theconfiguration of the optical scanning device 2010 will be described indetail later.

In the photosensitive drum 2030, an area in which image data is writtenmay be called an “effective scanning area”, an “image forming area”, oran “effective pixel area”.

The toner cartridge 2034 a stores therein black toner. The black toneris supplied to the developing roller 2033 a. The toner cartridge 2034 bstores therein cyan toner. The cyan toner is supplied to the developingroller 2033 b. The toner cartridge 2034 c stores therein magenta toner.The magenta toner is supplied to the developing roller 2033 c. The tonercartridge 2034 d stores therein yellow toner. The yellow toner issupplied to the developing roller 2033 d.

As the developing roller 2033 rotates, a light and uniform coat of tonerfrom the corresponding toner cartridge 2034 is applied to the surface ofthe developing roller 2033. The toner on the surface of the developingroller 2033, upon its contact with the surface of the correspondingphotosensitive drum 2030, is transferred only to portions irradiatedwith the light of the surface and adheres thereto. Specifically, thedeveloping roller 2033 causes the toner to adhere to the latent imageformed on the surface of the corresponding photosensitive drum 2030 tothereby visualize the latent image.

The transfer belt 2040 is trained over a belt rotating mechanism,rotating in a predetermined direction. The transfer belt 2040 has anouter surface contacting the surface of each of the photosensitive drums2030 a, 2030 b, 2030 c, 2030 d at a position opposite to the opticalscanning device 2010. In addition, the outer surface of the transferbelt 2040 contacts the transfer roller 2042.

An image to which the toner adheres on the surface of the photosensitivedrum 2030 (a toner image) is moved toward the transfer belt 2040 as thephotosensitive drum 2030 rotates. The toner images of yellow, magenta,cyan, and black are then transferred, in sequence, onto the surface ofthe transfer belt 2040 at predetermined timing and are superimposed oneon top of another to form a color image. The color image formed on thetransfer belt 2040 is moved toward the transfer roller 2042 as thetransfer belt 2040 rotates.

The paper feed tray 2060 stores therein recording sheets. The feedroller 2054 is disposed near the paper feed tray 2060. The feed roller2054 takes up the recording sheet, one at a time, from the paper feedtray 2060 and conveys the recording sheet to the pair of registrationrollers 2056.

The pair of registration rollers 2056 feeds the recording sheet toward anip between the transfer belt 2040 and the transfer roller 2042 atpredetermined timing. The color image on the transfer belt 2040 istransferred onto the recording sheet. The recording sheet onto which thecolor image has been transferred is fed to the fixing rollers 2050.

The fixing rollers 2050 apply heat and pressure to the recording sheet.This enables to fixing rollers 2050 to fix the toner on the recordingsheet. The recording sheet on which the toner has been fixed is fed ontothe discharge tray 2070 by way of the discharging rollers 2058 andstacked in sequence on the discharge tray 2070.

The cleaning unit 2031 removes toner remained (residual toner) on thesurface of the photosensitive drum 2030. The surface of thephotosensitive drum 2030 from which the residual toner has been removedreturns to a position facing the charging device 2032 again.

The density detector 2245 is disposed at a position on the negative Xside of the transfer belt 2040 (upstream of the fixing rollers 2050 inthe traveling direction of the transfer belt 2040 and downstream of thefour photosensitive drums 2030). Exemplarily, the density detector 2245includes three optical sensors 2245 a, 2245 b, 2245 c as illustrated inFIG. 2.

The optical sensor 2245 a is disposed at a position facing a positionnear the end portion on the negative Y side within the effective pixelarea in the transfer belt 2040 (on a first end side in the widthdirection of the transfer belt 2040). The optical sensor 2245 c isdisposed at a position facing a position near the end portion on thepositive Y side within the effective pixel area in the transfer belt2040 (on a second end side in the width direction of the transfer belt2040). The optical sensor 2245 b is disposed substantially at the centerbetween the optical sensor 2245 a and the optical sensor 2245 c in themain-scanning direction (at a central position in the width direction ofthe transfer belt 2040). In this specification, in the main-scanningdirection, the central position of the optical sensor 2245 a is denotedY1, the central position of the optical sensor 2245 b is denoted Y2, andthe central position of the optical sensor 2245 c is denoted Y3.

The optical sensors 2245 a, 2245 b, 2245 c each exemplarily include anLED 11, a regularly reflected light receiving element 12, and adiffusely reflected light receiving element 13 as illustrated in FIG. 3.Specifically, the LED 11 emits light (hereinafter referred to also asdetection light) toward the transfer belt 2040. The regularly reflectedlight receiving element 12 receives light reflected regularly from thetransfer belt 2040 or a toner pad on the transfer belt 2040. Thediffusely reflected light receiving element 13 receives light reflecteddiffusely from the transfer belt 2040 or the toner pad on the transferbelt 2040. Each of the regularly reflected light receiving element 12and the diffusely reflected light receiving element 13 outputs a signalcorresponding to the amount of received light (photoelectric conversionsignal).

The home position sensor 2246 a detects a home position in rotation ofthe photosensitive drum 2030 a. The home position sensor 2246 b detectsa home position in rotation of the photosensitive drum 2030 b. The homeposition sensor 2246 c detects a home position in rotation of thephotosensitive drum 2030 c. The home position sensor 2246 d detects ahome position in rotation of the photosensitive drum 2030 d.

FIG. 4 is a diagram illustrating the configuration of an optical systemof the optical scanning device 2010. FIG. 5 is a diagram illustrating anexemplary optical path from a light source 2200 a to a polygon mirror2104 and an exemplary optical path from a light source 2200 b to thepolygon mirror 2104. FIG. 6 is a diagram illustrating an exemplaryoptical path from a light source 2200 c to the polygon mirror 2104 andan exemplary optical path from a light source 2200 d to the polygonmirror 2104. FIG. 7 is a diagram illustrating an exemplary optical pathfrom the polygon mirror 2104 to the respective photosensitive drums2030.

The configuration of the optical system of the optical scanning device2010 will be described below. The optical scanning device 2010 includesas its optical system the four light sources 2200 a, 2200 b, 2200 c,2200 d, four coupling lenses 2201 a, 2201 b, 2201 c, 2201 d, fouraperture plates 2202 a, 2202 b, 2202 c, 2202 d, and four cylindricallenses 2204 a, 2204 b, 2204 c, 2204 d. The optical scanning device 2010further includes as the optical system the polygon mirror 2104, fourscanning lenses 2105 a, 2105 b, 2105 c, 2105 d, and six folding mirrors2106 a, 2106 b, 2106 c, 2106 d, 2108 b, 2108 c. These components aredisposed at respective predetermined positions in an optical housing.

The optical scanning device 2010 still further includes an electriccircuit which will be described with reference to FIG. 8 and onward.

The light sources 2200 a, 2200 b, 2200 c, 2200 d each include a surfaceemitting laser array in which a plurality of light emitting elements arearrayed two-dimensionally. The light emitting elements of the surfaceemitting laser array are arrayed so as to be equidistant from each otherwhen all light emitting elements are orthographically projected onto avirtual line extending in the direction corresponding to thesub-scanning direction. The light sources 2200 a, 2200 b, 2200 c, 2200 dare, for an example, each an exemplary vertical-cavity surface-emittinglaser (VCSEL).

The coupling lens 2201 a is disposed on the light path of a light beamemitted from the light source 2200 a, changing the light beam passingtherethrough to a substantially parallel light beam. The coupling lens2201 b is disposed on the light path of a light beam emitted from thelight source 2200 b, changing the light beam passing therethrough to asubstantially parallel light beam. The coupling lens 2201 c is disposedon the light path of a light beam emitted from the light source 2200 c,changing the light beam passing therethrough to a substantially parallellight beam. The coupling lens 2201 d is disposed on the light path of alight beam emitted from the light source 2200 d, changing the light beampassing therethrough to a substantially parallel light beam.

The aperture plate 2202 a has an aperture and shapes the light beam thathas passed through the coupling lens 2201 a. The aperture plate 2202 bhas an aperture and shapes the light beam that has passed through thecoupling lens 2201 b. The aperture plate 2202 c has an aperture andshapes the light beam that has passed through the coupling lens 2201 c.The aperture plate 2202 d has an aperture and shapes the light beam thathas passed through the coupling lens 2201 d.

The cylindrical lens 2204 a focuses the light beam that has passedthrough the aperture of the aperture plate 2202 a onto a position near adeflecting reflection surface of the polygon mirror 2104 along theZ-axis direction. The cylindrical lens 2204 b focuses the light beamthat has passed through the aperture of the aperture plate 2202 b onto aposition near the deflecting reflection surface of the polygon mirror2104 along the Z-axis direction. The cylindrical lens 2204 c focuses thelight beam that has passed through the aperture of the aperture plate2202 c onto a position near the deflecting reflection surface of thepolygon mirror 2104 along the Z-axis direction. The cylindrical lens2204 d focuses the light beam that has passed through the aperture ofthe aperture plate 2202 d onto a position near the deflecting reflectionsurface of the polygon mirror 2104 along the Z-axis direction.

An optical system comprising the coupling lens 2201 a, the apertureplate 2202 a, and the cylindrical lens 2204 a is a pre-deflector opticalsystem for the K station. An optical system comprising the coupling lens2201 b, the aperture plate 2202 b, and the cylindrical lens 2204 b is apre-deflector optical system for the C station. An optical systemcomprising the coupling lens 2201 c, the aperture plate 2202 c, and thecylindrical lens 2204 c is a pre-deflector optical system for the Mstation. An optical system comprising the coupling lens 2201 d, theaperture plate 2202 d, and the cylindrical lens 2204 d is apre-deflector optical system for the Y station.

The polygon mirror 2104 comprises a four-face mirror having a two-stagestructure rotating about an axis extending in parallel with the Z-axis,each face of the polygon mirror 2104 assuming a deflecting reflectionsurface. The polygon mirror 2104 is disposed such that the four-facemirror of a first stage (lower stage) deflects the light beam from thecylindrical lens 2204 b and the light beam from the cylindrical lens2204 c, while the four-face mirror of a second stage (upper stage)deflects the light beam from the cylindrical lens 2204 a and the lightbeam from the cylindrical lens 2204 d.

In addition, the light beam from the cylindrical lens 2204 a and thelight beam from the cylindrical lens 2204 b are deflected to thenegative X side of the polygon mirror 2104, while the light beam fromthe cylindrical lens 2204 c and the light beam from the cylindrical lens2204 d are deflected to the positive X side of the polygon mirror 2104.

The scanning lenses 2105 a, 2105 b, 2105 c, 2105 d each have an opticalpower that converges the light beam on a position near thephotosensitive drum 2030 and an optical power that causes an opticalspot to move on the surface of the photosensitive drum 2030 in themain-scanning direction at a constant speed as the polygon mirror 2104rotates.

The scanning lens 2105 a and the scanning lens 2105 b are disposed onthe negative X side of the polygon mirror 2104. The scanning lens 2105 cand the scanning lens 2105 d are disposed on the positive X side of thepolygon mirror 2104.

The scanning lens 2105 a and the scanning lens 2105 b are stacked in theZ-axis direction. The scanning lens 2105 b faces the four-face mirror ofthe first stage. The scanning lens 2105 a faces the four-face mirror ofthe second stage.

The scanning lens 2105 c and the scanning lens 2105 d are stacked in theZ-axis direction. The scanning lens 2105 c faces the four-face mirror ofthe first stage. The scanning lens 2105 d faces the four-face mirror ofthe second stage.

The photosensitive drum 2030 a is irradiated, via the scanning lens 2105a and the folding mirror 2106 a, with the light beam from thecylindrical lens 2204 a deflected by the polygon mirror 2104, whichforms an optical spot. The optical spot moves in the longitudinaldirection of the photosensitive drum 2030 a as the polygon mirror 2104rotates. Specifically, the optical spot scans the surface of thephotosensitive drum 2030 a. The direction in which the optical spotmoves at this time is the “main-scanning direction” in thephotosensitive drum 2030 a and the direction in which the photosensitivedrum 2030 a rotates is the “sub-scanning direction” in thephotosensitive drum 2030 a.

The photosensitive drum 2030 b is irradiated, via the scanning lens 2105b, the folding mirror 2106 b, and the folding mirror 2108 b, with thelight beam from the cylindrical lens 2204 b deflected by the polygonmirror 2104, which forms an optical spot. The optical spot moves in thelongitudinal direction of the photosensitive drum 2030 b as the polygonmirror 2104 rotates. Specifically, the optical spot scans the surface ofthe photosensitive drum 2030 b. The direction in which the optical spotmoves at this time is the “main-scanning direction” in thephotosensitive drum 2030 b and the direction in which the photosensitivedrum 2030 b rotates is the “sub-scanning direction” in thephotosensitive drum 2030 b.

The photosensitive drum 2030 c is irradiated, via the scanning lens 2105c, the folding mirror 2106 c, and the folding mirror 2108 c, with thelight beam from the cylindrical lens 2204 c deflected by the polygonmirror 2104, which forms an optical spot. The optical spot moves in thelongitudinal direction of the photosensitive drum 2030 c as the polygonmirror 2104 rotates. Specifically, the optical spot scans the surface ofthe photosensitive drum 2030 c. The direction in which the optical spotmoves at this time is the “main-scanning direction” in thephotosensitive drum 2030 c and the direction in which the photosensitivedrum 2030 c rotates is the “sub-scanning direction” in thephotosensitive drum 2030 c.

The photosensitive drum 2030 d is irradiated, via the scanning lens 2105d and the folding mirror 2106 d, with the light beam from thecylindrical lens 2204 d deflected by the polygon mirror 2104, whichforms an optical spot. The optical spot moves in the longitudinaldirection of the photosensitive drum 2030 d as the polygon mirror 2104rotates. Specifically, the optical spot scans the surface of thephotosensitive drum 2030 d. The direction in which the optical spotmoves at this time is the “main-scanning direction” in thephotosensitive drum 2030 d and the direction in which the photosensitivedrum 2030 d rotates is the “sub-scanning direction” in thephotosensitive drum 2030 d.

The folding mirrors 2106 a, 2106 b, 2106 c, 2106 d, 2108 b, 2108 c aredisposed such that each has an optical path length between the polygonmirror 2104 and the corresponding photosensitive drum 2030 identical toeach other and the position of incidence and the incident angle of thelight beam at the corresponding photosensitive drum 2030 are identicalto each other.

The optical system disposed along the optical path between the polygonmirror 2104 and the photosensitive drum 2030 is also referred to as ascanning optical system. In the embodiment, the scanning lens 2105 a andthe folding mirror 2106 a constitute a scanning optical system for the Kstation. Similarly, the scanning lens 2105 b and the two folding mirrors2106 b, 2108 b constitute a scanning optical system for the C station.The scanning lens 2105 c and the two folding mirrors 2106 c, 2108 cconstitute a scanning optical system for the M station. The scanninglens 2105 d and the folding mirror 2106 d constitute a scanning opticalsystem for the Y station. In each of these scanning optical systems, thescanning lens 2105 a, 2105 b, 2105 c, or 2105 d may comprise a pluralityof lenses.

FIG. 8 is a diagram illustrating the configuration of an electricalsystem of the optical scanning device 2010. The optical scanning device2010 includes as its electrical system an interface unit 3101, an imageprocessing unit 3102, and a drive control unit 3103.

The interface unit 3101 acquires, from the printer control device 2090,image data transferred from the host device (e.g., a computer). Theinterface unit 3101 transfers the acquired image data to the imageprocessing unit 3102 downstream thereof.

In the embodiment, the interface unit 3101 acquires image data in theRGB format having a resolution of 1200 dpi and consisting of eight bitsand transfers the image data to the image processing unit 3102.

The image processing unit 3102, having acquired the image data from theinterface unit 3101, converts the image data to color image datacorresponding to the applicable printing system. Exemplarily, the imageprocessing unit 3102 converts the image data in the RGB format to imagedata of the tandem type (CMYK format). In addition to the data formatconversion, the image processing unit 3102 performs image processingwith the aim of, for example, improving image quality of the image data.

In this embodiment, the image processing unit 3102 outputs image data inthe CMYK format having a resolution of 1200 dpi and consisting of twobits. The image data output from the image processing unit 3102 may haveany resolution other than 1200 dpi. The resolution of the image dataoutput from the image processing unit 3102 is referred to as a firstresolution.

The drive control unit 3103 acquires, from the image processing unit3102, image data having the first resolution, and converts the imagedata to color image data having a second resolution corresponding tolight source driving. The second resolution is higher than the firstresolution. In this embodiment, the drive control unit 3103 converts theimage data to one in the CMYK format having a resolution of 4800 dpi andconsisting of one bit.

Additionally, the drive control unit 3103 modulates the image data to aclock signal that indicates pixel light-emitting timing, thus generatingan independent modulation signal for each color. The drive control unit3103 drives and causes each of the light sources 2200 a, 2200 b, 2200 c,2200 d to emit light according to the modulation signal associated withthe corresponding color.

The drive control unit 3103 is exemplarily a single-chip IC circuitdisposed near the light sources 2200 a, 2200 b, 2200 c, 2200 d. Theimage processing unit 3102 and the interface unit 3101 are disposedfarther away from the light sources 2200 a, 2200 b, 2200 c, 2200 drelative to the drive control unit 3103. The image processing unit 3102and the drive control unit 3103 are connected with a cable 3104.

The optical scanning device 2010 having the arrangements as describedabove can form a latent image by causing the light sources 2200 a, 2200b, 2200 c, 2200 d to emit light corresponding to the image data.

FIG. 9 is a diagram illustrating the configuration of the interface unit3101. The interface unit 3101 exemplarily includes a flash memory 3211,a RAM 3212, an IF circuit 3213, and a CPU 3214. The flash memory 3211,the RAM 3212, the IF circuit 3213, and the CPU 3214 are connected toeach other by a bus.

The flash memory 3211 stores therein a computer program to be executedby the CPU 3214 and various types of data required by the CPU 3214 forexecuting the program. The RAM 3212 is a work storage area used by theCPU 3214 to execute the program. The IF circuit 3213 performsbi-directional communications with the printer control device 2090.

The CPU 3214 operates according to the program stored in the flashmemory 3211, thus generally controlling the optical scanning device2010. The interface unit 3101 configured as described above transfersthe image data (in the RGB format having a resolution of 1200 dpi andconsisting of eight bits) transmitted from the printer control device2090 to the image processing unit 3102.

FIG. 10 is a diagram illustrating the configuration of the imageprocessing unit 3102. The image processing unit 3102 includes a colorconverter 3221, an ink generator 3222, a gamma corrector 3223, aquasi-halftone processor 3224, and a first image processor 3225.

The color converter 3221 converts the image data in the RGB format toimage data in a CMY format. The ink generator 3222 generates a blackcomponent from the image data in the CMY format generated by the colorconverter 3221 to thereby generate image data in the CMYK format.

The gamma corrector 3223 uses, for example, a table to subject the levelof each color to linear conversion. The quasi-halftone processor 3224uses, for example, a dithering technique to process halftones, therebyreducing the number of gradations of the image data.

The first image processor 3225 performs image processing on the imagedata output from the quasi-halftone processor 3224 with the aim of, forexample, improving image quality. The first image processor 3225 usesfiltering, pattern matching, or the like to detect, within the image, aspecific area for which image quality is to be improved and performspredetermined image processing on the detected image area.

Specific examples of the processing performed by the first imageprocessor 3225 will further be described in detail with reference toFIGS. 12A, 12B, 13A, 13B, and 14. In addition, the first image processor3225 performs the image processing on an area different from thatprocessed in image processing performed by the drive control unit 3103at a later stage and using parameters different from those used in theimage processing performed by the drive control unit 3103. Thedifferences will be described in detail later.

The image processing unit 3102 as described above outputs the image datain the CMYK format having the first resolution (e.g., 1200 dpi) andconsisting of two bits to the drive control unit 3103. The imageprocessing unit 3102 may be achieved by hardware partially or entirelyor by a CPU executing a software program.

FIG. 11 is a diagram illustrating the configuration of the drive controlunit 3103. The drive control unit 3103 includes a resolution converter3231, a clock generator 3232, a modulation signal generator 3233, alight source driver 3234, and a second image processor 3235.

The resolution converter 3231 acquires image data having the firstresolution from the image processing unit 3102 and converts the imagedata to image having the second resolution that is higher than the firstresolution. In the embodiment, the resolution converter 3231 convertsthe image data in the CMYK format having a resolution of 1200 dpi andconsisting of two bits to image data in the CMYK format having aresolution of 4800 dpi and consisting of one bit.

Specifically, the resolution converter 3231 quadruples the resolution byconverting one dot (two bits (four gradations)) of 1200-dpi image datato four dots (one bit) of 4800-dpi image data. It is noted that theresolution converter 3231 may convert image data to that of anygradations, as long as the conversion process converts image data with aresolution N (N being a natural number) to image data with a resolutionof m×N (m being 2 or any other natural number more than 2).

The clock generator 3232 generates a clock signal that indicates thepixel light-emitting timing. The clock signal can be phase-modulatedwith a resolution of ⅛ clock, for example.

The modulation signal generator 3233 modulates image data of each colorto a corresponding clock signal to thereby generate an independentmodulation signal for the color. In the embodiment, the modulationsignal generator 3233 generates a modulation signal for each color of C,M, Y, and K. Additionally, the modulation signal generator 3233modulates, for each color, the image data to a clock signal insynchronism with write start timing based on the angular position ofrotation of the photosensitive drum 2030. The modulation signalgenerator 3233 then supplies the independent modulation signal for eachcolor to the light source driver 3234.

The light source driver 3234 drives a corresponding one of the lightsources 2200 a, 2200 b, 2200 c, 2200 d according to the independentmodulation signal for each color output from the modulation signalgenerator 3233. This enables the light source driver 3234 to make eachof the light sources 2200 a, 2200 b, 2200 c, 2200 d emit light with anintensity corresponding to the modulation signal.

The second image processor 3235 performs image processing for the imagedata having the second resolution (e.g., 4800 dpi) to be modulated to amodulation signal. The second image processor 3235 exemplarily includesa pattern matcher 3241 and a corrector 3242.

The pattern matcher 3241 detects, of the image data, an image area to besubject to processing by the second image processor 3235. Exemplarily,the pattern matcher 3241 detects from the image data having the secondresolution an area with a space component close to that of a previouslyregistered image pattern. Alternatively, the pattern matcher 3241 mayperform filtering for the image data having the second resolution tothereby detect an area with a frequency component close to that of apreviously registered image pattern.

The corrector 3242 corrects the detected image area through imageprocessing. For example, the corrector 3242 may perform image processingfor the image data before modulation. Alternatively, the corrector 3242may even perform image processing for the image data by adjusting signalintensity by, for example, changing the pulse width of the modulationsignal during the modulation.

As briefly noted earlier, the first image processor 3225 of the imageprocessing unit 3102 and the second image processor 3235 of the drivecontrol unit 3103 perform image processing using processing parametersdifferent from each other or relative to areas subject to image dataprocessing different from each other.

For example, the first image processor 3225 performs image processingrendering a coarseness level coarser than a predetermined coarsenesslevel for the image data having the first resolution (e.g., 1200 dpi).At this time, the second image processor 3235 performs image processingrendering a fineness level finer than a predetermined fineness level forthe image data having the second resolution (e.g., 4800 dpi). Thisallows the first image processor 3225 to perform a coarse adjustment andthe second image processor 3235 to perform a fine adjustment relative toan identical image area.

Exemplarily, the first image processor 3225 performs image processingfor objects (e.g., characters or graphics) equal in size to or larger insize than a predetermined size on the image data having the firstresolution (e.g., 1200 dpi). In this case, the second image processor3235 performs image processing for objects (e.g., characters orgraphics) smaller in size than the predetermined size, the objects notbeing subject to the image processing by the first image processor 3225.Exemplarily, the second image processor 3235 performs its imageprocessing for at least part of a pattern of predetermined charactershaving a predetermined size or smaller. This allows the first imageprocessor 3225 to perform its image processing for a coarse image areaand the second image processor 3235 to perform the same image processingas that of the first image processor 3225 for a fine image area.

Thus, the color printer 2000 performs image processing for a minutepattern or fine image processing on high-resolution image data, whichenables the color printer 2000 to form an image with high quality.Furthermore, the color printer 2000 performs image processing for arelatively large pattern or relatively coarse image processing onlow-resolution image data. This reduces processing load on the drivecontrol unit 3103, while reducing the amount of data transferred fromthe image processing unit 3102 to the drive control unit 3103.

In detecting the object, such as a character or a graphic figure, to besubjected to image processing, if it is difficult for the first imageprocessor 3225 and the second image processor 3235 to detect an entireobject from the image data, at least part of the character or graphicfigure may be detected by, for example, pattern matching. For example,the first image processor 3225 and the second image processor 3235register at least a characteristic shape pattern of a predeterminedcharacter or graphic figure in advance and detect the whole or part ofthe character by pattern matching. The first image processor 3225 andthe second image processor 3235 then perform the image processing on thedetected whole or part of the character.

FIG. 12A is a diagram illustrating an exemplary 5-point white-on-blackinverted character and exemplary enlarging steps in units of 1200 dpi.FIG. 12B is a diagram illustrating an exemplary 3-point white-on-blackinverted character and exemplary enlarging steps in units of 4800 dpi.The first image processor 3225 and the second image processor 3235detect, for example, a white (blank) portion that is represented byblanking out the shape of an object (e.g., a character or a graphicfigure) from a background color as illustrated in FIGS. 12A and 12Bthrough matching between the space component or the frequency componentof the image data and a previously registered pattern. Then, the firstimage processor 3225 and the second image processor 3235 perform stepsof enlarging a white (blank) part in the detected white (blank) portion.

This enables the first image processor 3225 and the second imageprocessor 3235 to form a high-quality image by minimizing a disadvantagein electrophotographic printing of aggravated reproducibility due tocollapsed fine lines.

In performing the steps of enlarging the white part, the first imageprocessor 3225 enlarges, relative to the white-on-black invertedcharacter of a predetermined first size or larger (e.g., 5 points orlarger), the white part in units of the first resolution (e.g., in unitsof 1200 dpi), but not relative to the white-on-black inverted charactersmaller than the first size (e.g., smaller than 5 points), asillustrated in FIG. 12A.

Alternatively, as illustrated in FIG. 12B, the second image processor3235 enlarges, relative to the white-on-black inverted character notsubjected to the image processing performed by the first image processor3225, specifically, the white-on-black inverted character smaller thanthe first size (e.g., smaller than 5 points), the white part in units ofthe second resolution (e.g., in units of 4800 dpi), but not relative tothe white-on-black inverted character equal to or larger than the firstsize (e.g., 5 points or larger).

This allows the first image processor 3225 to perform its imageprocessing for a relatively coarse image area and the second imageprocessor 3235 to perform the same image processing as that of the firstimage processor 3225 for a relatively fine image area. It is noted that,in this case, the second image processor 3235 may enlarge the white partby, for example, changing the pulse width of the modulation signal ofparts surrounding the white part.

FIG. 13A is a diagram illustrating exemplary thinning steps in units of1200 dpi. FIG. 13B is a diagram illustrating exemplary thinning steps inunits of 4800 dpi. Exemplarily, the first image processor 3225 detects aline-shaped object as illustrated in FIG. 13A through matching betweenthe space component or the frequency component of the image data and apreviously registered pattern in units of the first resolution (e.g., inunits of 1200 dpi). The first image processor 3225 then performs a stepof changing the width of the line (e.g., thinning) in units of the firstresolution (e.g., in units of 1200 dpi) relative to the detectedline-shaped object. The second image processor 3235 detects aline-shaped object as illustrated in FIG. 13B through matching betweenthe space component or the frequency component of the image data and apreviously registered pattern in units of the second resolution (e.g.,in units of 4800 dpi). The second image processor 3235 then performs astep of changing the width of the line (e.g., thinning) in units of thesecond resolution (e.g., in units of 4800 dpi) relative to the detectedline-shaped object.

This enables the first image processor 3225 and the second imageprocessor 3235 to form a high-quality image by minimizing a disadvantageof character thickening resulting from electrophotographic printing.

In performing the step of changing the width of the line as describedabove, the first image processor 3225 changes the width of a line with apredetermined width or larger (e.g., 5 dots or more at 1200 dpi), butnot for a line with a width smaller than the predetermined width. Thesecond image processor 3235 changes, relative to the line with a widthnot subjected to the step performed by the first image processor 3225,specifically, the width of the line with a width smaller than thepredetermined width (e.g., a line of less than 20 dots at 4800 dpi), butnot for a line with the predetermined width or larger.

This allows the first image processor 3225 to perform its imageprocessing for a relatively coarse image area and the second imageprocessor 3235 to perform the same image processing as that of the firstimage processor 3225 for a relatively fine image area. It is noted that,in this case, the second image processor 3235 may narrow edges of theline by, for example, changing the pulse width of the modulation signalof line edge portions.

FIG. 14 is a diagram illustrating exemplary smoothing steps. Asillustrated in FIG. 14, the first image processor 3225 and the secondimage processor 3235 exemplarily detect a line-shaped object drawn in anoblique direction relative to an array of dots of image data throughmatching between the space component or the frequency component of theimage data and a previously registered pattern. The first imageprocessor 3225 and the second image processor 3235 then performs asmoothing step that smoothes edges of the detected oblique line.

This enables the first image processor 3225 and the second imageprocessor 3235 to improve line reproducibility, thereby forming ahigh-quality image.

In performing the smoothing step for the oblique line as describedabove, the first image processor 3225 smoothes the oblique line in unitsof predetermined pixels (e.g., one dot at 1200 dpi). The second imageprocessor 3235 smoothes the oblique line in units of pixels (e.g., onedot at 4800 dpi) with which an oblique line that cannot be smoothed inunits of pixels applicable to the first image processor 3225 can besmoothed.

Assume, for example, that the first image processor 3225 smoothes theoblique line in units of one dot at 1200 dpi as illustrated by figure(A) in FIG. 14. In this case, the second image processor 3235 firstthickens the oblique line as illustrated by figure (B) in FIG. 14 andthen smoothes the thickened oblique line in units of one dot at 4800 dpias illustrated by figure (C) in FIG. 14.

As such, the first image processor 3225 and the second image processor3235 can further improve line reproducibility by performing thethickening and smoothing steps for the oblique line. This allows thefirst image processor 3225 to perform coarse adjustments and the secondimage processor 3235 to perform fine adjustments relative to the sameimage area.

FIG. 15 is a diagram illustrating a modification of the drive controlunit 3103 of the optical scanning device 2010. The drive control unit3103 may receive object information together with the image data fromthe image processing unit 3102. The object information indicates, foreach image area (e.g., for each pixel dot) of the image data, the typeof an object of the image area.

If, for example, the corresponding dot is part of a character, theobject information indicates an attribute that represents a “character”.Alternatively, if the corresponding dot is part of a graphic figure, theobject information indicates an attribute that represents a “graphicfigure”. If the corresponding dot is part of a photo, the objectinformation indicates an attribute that represents a “photo”.

According to a specific detail of the received object information, thesecond image processor 3235 determines whether to perform imageprocessing. If, for example, the received object information indicatesthe attribute that represents a “character”, and if, for example, thearea in question is subject to processing for a white-on-black invertedcharacter, the second image processor 3235 performs the imageprocessing. If the received object information indicates an attributerepresenting one other than a character, the second image processor 3235does not perform image processing for the area subject to processing forthe white-on-black inverted character.

As described above, the second image processor 3235 can improve imagequality with even higher accuracy by determining whether to performimage processing using the object information.

The present invention achieves an advantageous effect of performingimage processing at high resolutions without increasing the amount ofoptical image data to be transferred.

Although the invention has been described with respect to specificembodiments for a complete and clear disclosure, the appended claims arenot to be thus limited but are to be construed as embodying allmodifications and alternative constructions that may occur to oneskilled in the art that fairly fall within the basic teaching herein setforth.

What is claimed is:
 1. An image forming apparatus that forms an imageaccording to light emitted from a light source, the image formingapparatus comprising: a first image processor configured to performsimage processing on image data having a first resolution and output theresulting image data; a resolution converter configured to acquire theimage data having the first resolution output from the first imageprocessor and convert the image data to image data having a secondresolution that is higher than the first resolution; a modulation signalgenerator configured to modulate the image data having the secondresolution according to a clock signal to thereby generate a modulationsignal; a light source driver configured to drive the light sourceaccording to the modulation signal; and a second image processorconfigured to perform image processing on the image data having thesecond resolution to be modulated to the modulation signal.
 2. The imageforming apparatus according to claim 1, wherein the first imageprocessor performs image processing that renders a coarseness levelcoarser than a predetermined coarseness level on the image data havingthe first resolution, and the second image processor performs imageprocessing that renders a fineness level finer than a predeterminedfineness level on the image data having the second resolution.
 3. Theimage forming apparatus according to claim 1, wherein the first imageprocessor performs image processing for at least part of a pattern of anobject having a predetermined size or larger on the image data havingthe first resolution, and the second image processor performs imageprocessing for at least part of a pattern of an object smaller than thepredetermined size on the image data having the second resolution. 4.The image forming apparatus according to claim 3, wherein the secondimage processor performs image processing for at least part of a patternof a predetermined character having the predetermined size or smaller.5. The image forming apparatus according to claim 1, wherein the secondimage processor acquires object information that indicates a type of anobject in each image area of image data and changes over types ofprocessing performed on the image area according to the acquired objectinformation.
 6. The image forming apparatus according to claim 1,wherein the second image processor detects, from the image data havingthe second resolution, a white-on-black inverted portion represented byblanking out an object smaller than a first size and enlarges a whitepart of the detected white-on-black inverted portion.
 7. The imageforming apparatus according to claim 1, wherein the second imageprocessor detects, from the image data having the second resolution, aline having a predetermined width or smaller and changes a width of thedetected line.
 8. The image forming apparatus according to claim 1,wherein the second image processor detects, from the image data havingthe second resolution, a line having a predetermined width or smaller,the line being drawn in an oblique direction relative to an array ofdots of the image data, and smoothes edges of the detected line.
 9. Theimage forming apparatus according to claim 1, wherein the light sourcecomprises a vertical-cavity surface-emitting laser.